INDUCED SEISMICITYIn-Situ Laboratories for Understanding

Fluid-Driven Fracture Systems

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Cluster 1

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Depth view at fracture face Depth view at fracture face

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SoultzSoultz

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Soultz, Shallow Cluster Soultz, Deep Cluster

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Discrete fracture plane orientations imaged from the Fenton Hill reservoir. The distinct fracture edges resolved correlate with significant fracture-plane intersections.

A sub-cluster of events from the Fenton Hill HDR reservoir. Original locations (left) and refined locations (right) after obtaining high-precision arrival-time data. In the face-on view (bottom right) sharp, straight edges bound the location pattern.

Two event sub-clusters each delineating the intersection of two planes. Looking along line of intersection (top) and face-on views (bottom). Linear trends can be seen along fracture faces (bottom). The event sequences indicate early fluid invasion along linear trends.

Location of the Soultz HDR site.

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Microearthquakes induced by fluid injection provide a means to study fluid migration, fracture creation and faulting, with an optimal combination of resolution and subsurface coverage. Supplemental information often available with induced experiments includes borehole imagery, stress measurements and fluid pressure and flow data, information that greatly

aids in understanding similar earth phenomena where little independent information exists. Further, the nature of induced microseismicity enables exceedingly high-precision locations to be obtained, revealing discrete fracture networks and fine-scale rupture sequence patterns with remarkable clarity.

Poles to planes

Cluster edgeorientations

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Composite fault plane solutions indicate thrust displace-ment along the mapped, low-angle fractures. Induced stress changes are very small (<0.1 MPa).

Seismically active fractures correlate with previously depleted zones; current productive zone is aseismic.

Event rate changes correlate with production rate changes. Seismic response lags production by 2–3 weeks.

Locations of microearthquakes induced during a hydraulic fracture stimulation treatment in a tight-gas sand reservoir. The dashed lines mark the perforated interval.

Re-picking the data in a spatial sequence revealed a remarkable similarity of adjacent-source waveforms and evolution of waveforms along the treatment length. Systematic polarity changes and phase amplitude ratios indicate uniformity of focal mechanisms along the entire fracture.

Two stimulation treatments were mapped in the Giddings Austin Chalk field. Event locations at Cook's Point show greater width development and less length than the Matcek Ranch stimulation.

The wider stimulation zone imaged at Cook's Point is correlated with a greater production response.

Reflected phase off the base of the Eagleford allowed location depths to be constrained.

The fracture growth is characterized by systematic space-time patterns of seismicity along the treatment length.

Refined images after obtaining higher-precision arrival time data via waveform correlation.

Horizontal bands of seismicity correlate closely with the treatment interval perforation schedule and proppant placement.

Above-reservoir casing deformations have been observed in several wells in the southeast Geysers field. Deformation is concentrated over short depth intervals at the upper and lower contacts of a serpentine unit.

Both the above-reservoir and the deeper reservoir seismicity show correlations with reservoir injection activity in the study area.

Small magnitude events were detected above the reservoir by placing a downhole receiver array within the upper 800 feet of an abandoned well. Event occurence terminates near the base of the serpentine unit.

Fracture planes are delineated by production-induced events occuring over a 6-month monitor period.

Cumulative production and microearthquake event counts Originallocations

LeftLateralStrikeSlip

RightLateralStrikeSlip

High-precision locations

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HYDRAULIC FRACTURE IMAGINGCarthage Gas Field, East Texas

PRODUCTIVE FRACTURE IMAGINGClinton County, Kentucky

WELL-BORE DEFORMATIONThe Geysers, California

Austin Chalk, Texas

P

TT

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Seismicity Induced byPetroleum and Geothermal

ActivitiesJim Rutledge and Scott Phillips

Los Alamos National Laboratory

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Fenton HillExperiment 2032

log10 Event Density (m-2)

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Introduction

Microseismic fracture mapping at Los Alamos started with the Hot Dry Rock geothermal project. The fractured reservoir volumes created and stimulated during massive hydraulic fracturing were imaged using the microearth-quakes induced by injection. Seismic receivers need to be placed downhole at or near reservoir depth to detect the small magnitude events. In the 1990s we began to apply these techniques to oil and gas problems. In addition to the now well-known application of hydraulic fracture imaging, other applications include mapping natural, conductive fractures affected by extraction and pressure recovery operations, as well as monitoring and characterizing reservoir and overburden deformation induced by production.

Applications

Hydraulic fracture imaging

Natural fracture imaging - production induced - water-flood or CO2-flood front mapping

Monitoring waste containment and flow paths - hydraulic fracture injection disposal - CO2 geologic sequestration - thermal induced stress changes

Monitoring and characterizing reservoir deformation and subsidence